The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Selective Catalytic Reduction (SCR) of NOx using ammonia (NH3) as a reductant is well established for NOx emissions reduction on stationary sources and mobile applications such as diesel engines. In the SCR process, NOx reacts with the reductant, such as a pure anhydrous ammonia, aqueous ammonia, or urea, which is injected by a dosing system into the flue or exhaust gas stream to be absorbed onto a SCR catalyst. The injected dosing agent (e.g. urea) breaks down to form ammonia (NH3), which is the reductant utilized to react with the NOx. The following exemplary, chemical relationships describe the NOx reduction:4NO+4NH3+O2→4N2+6H2O4NH3+2NO+2NO→4N2+6H2O3NO2+4NH3→3.5N2+6H2OThe SCR process significantly reduces NOx, forming water vapor (H2O) and nitrogen gas (N2).
The ability of the SCR catalyst to absorb NH3 is a function of temperature, with storage capacity decreasing with increasing SCR catalyst temperatures. To attain good NOx conversion efficiency of the SCR catalyst, especially at a lower range of operating temperatures, the SCR catalyst must be maintained with an excess amount of NH3 stored in the catalyst. However, as SCR catalyst temperature increases, stored NH3 may be released from the catalyst due to decreasing storage capacity. NH3 release is most sensitive to SCR catalyst temperature transients, where a rapid temperature elevation suddenly reduces the storage capacity, freeing stored NH3. If the NOx input rate is not sufficient to consume the freed NH3 during the thermal transient, the freed NH3 is released out the rear of the SCR catalyst. Release of NH3 affects the calculation of NOx conversion efficiency as the NOx sensor downstream of the SCR catalyst detects the released NH3 as NOx. Furthermore, if a catalyzed diesel particulate filter is present downstream of the SCR catalyst and is sufficiently hot, the NH3 may be oxidized to NOx, thereby increasing the tailpipe NOx.